This invention relates to a solid electrolyte capacitor in which the solid electrolyte is an electroconductive polyaniline doped with a disulfonic acid, and more particularly to the use of a group of novel disulfonic acids each as the dopant in the polyaniline solid electrolyte.
It is well known that polyaniline provides an electroconductive polymer by doping with a protonic acid which can be selected from various organic and inorganic acids. Herein, the term "polyaniline" means a polymer of aniline or an aniline derivative (substituted aniline). Polyaniline can be made relatively high in conductivity, and an important advantage of polyaniline over other electroconductive polymers represented by polypyrrole is very good stability in the air.
Recently it has been developed to use electroconductive polyanilines in solid electrolyte capacitors as the solid electrolyte. In fact, by using a polyaniline solid electrolyte it is possible to produce a small-sized capacitor that is relatively large in capacitance and fairly good in heat resistance and exhibits good capacitor characteristics in a high-frequency region.
Various kinds of protonic acids have been used in polyanilines in solid electrolyte capacitors, and recently there is a trend toward the use of organic sulfonic acids. For example, JP 62-29124 A and JP 64-24410 A show using arylsulfonic acids such as benzenesulfonic acid, toluenesulfonic acid and naphthalenedisulfonic acid, and JP 5-83167 shows making a selection from arylsulfonic acids, arylenedisulfonic acids, alkylsulfonic acids and alkyelenedisulfonic acids.
The important characteristics of a polyaniline solid electrolyte such as conductivity, heat resistance and humidity resistance depend greatly on the chemical structure of the sulfonic acid used as the dopant. As to heat resistance of the solid electrolyte in a practical capacitor, a matter of serious concern is the endurance to soldering temperatures at the time of packaging the capacitor or mounting the capacitor to a printed circuit board. The soldering temperatures range from about 230.degree. C. to about 260.degree. C. In general, polyaniline solid electrolytes doped with either an alkylmonosulfonic acid or an arylmonosulfonic acid are insufficient in humidity resistance and high-temperature endurance. The high-temperature endurance can be improved by using a disulfonic acid, and in this regard arylenedisulfonic acids are better than alkylenedisulfonic acids. There is a possibility of further improving both humidity resistance and high-temperature endurance by using a polysulfonic acid. However, still there is a demand for a superior dopant for polyaniline in order to obtain a polyaniline solid electrolyte which is fully satisfactory in respect of conductivity, capability of realizing large capacitance, high-temperature endurance and humidity resistance.
In the fabrication of a solid electrolyte capacitor using polyaniline, it is usual to form the solid electrolyte layer of polyaniline by polymerizing aniline over a dielectric oxide film on the anode by using an oxidant in the presence of a sulfonic acid.
Polyanilines are generally poorly soluble in conventional organic solvents. However, it is possible to obtain a soluble polyaniline by a special method as reported by Abe et al., J. of Chem. Soc., Chemical Communications (1989), pp. 1736-1738. JP 5-41338 A relates to the fabrication of a solid electrolyte capacitor and shows forming a polyaniline solid electrolyte layer by first forming an undoped polyaniline layer by applying a solution of a soluble polyaniline onto the dielectric oxide film on the anode and removing the solvent and thereafter doping the polyaniline layer with a disulfonic acid or a polysulfonic acid. The post-doped polyaniline is very good in humidity resistance. This method may be practicable when the anode body is a metal foil (usually aluminum foil) which is etched for enlargement of surface area, but this method is impracticable when a sintered pellet (usually of tantalum) is used as the anode because high viscosity of the polyaniline solution hinders the solution from intruding into the micropores in the sintered pellet.